Systems and methods for nox sensor diagnostics

A method, comprising:

• meeting phase one conditions of a system-out NOx sensor diagnostic procedure by determining an exhaust gas temperature of an exhaust gas produced by an internal combustion engine satisfies an exhaust gas temperature target while injecting a reductant into the exhaust gas so that a selective catalytic reduction (SCR) catalyst receiving the exhaust gas operates to meet oxides of nitrogen (NOx) emissions criteria;
• after meeting phase one conditions, meeting phase two conditions of the diagnostic procedure by determining the exhaust gas temperature satisfies the exhaust gas temperature target and an exhaust gas flow of the exhaust gas satisfies an exhaust gas flow target;
• after meeting phase two conditions, determining a diagnostic dosing command in response to an ammonia to NOx ratio (ANR) rate that is less than a stoichiometric ANR rate;
• depleting reductant storage of the SCR catalyst with the exhaust gas from the internal combustion engine while injecting reductant into the exhaust gas according to the diagnostic dosing command;
• after depleting reductant storage of the SCR catalyst, meeting phase three conditions of the diagnostic procedure by determining the exhaust gas flow satisfies the exhaust gas flow target;
• after meeting phase three conditions, determining a gain error of a system-out NOx sensor located downstream of the SCR catalyst while injecting reductant into the exhaust gas according to the diagnostic dosing command;
• and
• determining a fault condition of the system-out NOx sensor in response to the comparing.

A system, comprising:

• an internal combustion engine operable to produce an exhaust gas;
• an exhaust path connected to the internal combustion engine, the exhaust path including a selective catalytic reduction (SCR) catalyst and a system-out oxide of nitrogen (NOx) sensor downstream of the SCR catalyst;
• a reductant system including a reductant storage tank and a reductant injector connected to the reductant storage tank that is configured to receive reductant from the reductant storage tank and supply reductant to the exhaust path upstream of the SCR catalyst;
• a controller, including:
• a diagnostic staging module configured to determine a temperature of the exhaust gas satisfies an exhaust gas temperature target and to determine a flow of the exhaust gas satisfies an exhaust gas flow target;
• a diagnostic dosing rate module configured to provide a diagnostic dosing command to the reductant injector according to an ammonia to NOx ratio (ANR) rate that is substantially less than a stoichiometric ANR rate when the exhaust gas temperature and exhaust gas flow targets are satisfied, wherein the diagnostic dosing rate module is further configured to provide the diagnostic dosing command to inject reductant during depletion of reductant in the exhaust path with the exhaust gas;
• a system-out NOx sensor diagnostic module configured to, after depletion of the reductant, compare a measured system-out NOx amount from the system-out NOx sensor with an estimated system-out NOx and to provide a fault setting for the system-out NOx sensor in response to the comparison.

A method, comprising:

• operating an internal combustion engine in a first mode that produces exhaust gas with oxides of nitrogen (NOx) into an exhaust system including a selective catalytic reduction (SCR) catalyst;
• setting a diagnostic dosing command for injection of a reductant into the exhaust system at an ammonia to NOx ratio (ANR) rate that is substantially less than a stoichiometric ANR rate;
• operating the internal combustion engine in a second mode in which NOx production is substantially reduced from the first mode while injecting reductant into the exhaust gas upstream of the SCR catalyst according to the diagnostic dosing command;
• and
• determining a fault condition of a system-out NOx sensor downstream of the SCR catalyst while operating the internal combustion engine in the second mode and injecting reductant according to the diagnostic dosing command.